1. Field of the Invention
The present invention relates to a wire cut electrical discharge machine(EDM). More particularly, it relates to a method and device for controlling discharge the current slope of wire cut electrical discharge machine.
2. Description of the Prior Art
Referring to FIG. 1, conventional wire cut electrical discharge machine(EDM) as disclosed in Taiwan Utility Model Patent Publication No. 82108873, as shown in FIG. 1, comprises a power source 1, a capacitor 2, electrode wires 3 including lines L1.about.Ln combined with point "a" line of a loop circuit, a wire electrode 4 of the wire cut EDM and a workpiece 5. To maintain a stable electrical discharge state, the wire cut EDM controls the discharging conditions by sending the voltage condition of discharge gap 6 to an electrical discharge detecting circuit 7 to judge whether it is in one of the normal discharge or the arc discharge conditions, then deciding the pulse width of discharging duration through a timing controller circuit 8, driving a set of switches 10 by a driver circuit 9 which receives the output of the timing controller circuit 8 to control the turn-on time of SW-1.about.SW-n of switches 10 respectively so that the pulse width of the normal discharge is twice that of the arc discharge. The main discharge loop circuit of the conventional wire cut EDM has no current-limited resistor, and its machining current slope which is constant after finishing configuration is determined by the inductance of the electrode wire 3. A large machining current slope will obtain a faster machining rate and therefore can be more efficient, but it is more possible to cause wire electrode breakage is excess current peak and current density of arc discharge arc provided. The method of detecting electrical discharge of the conventional wire cut EDM is to provide a detecting voltage between the wire electrode 4 and the workpiece 5, and checking the gap voltage between the wire electrode 4 and the workpiece 5 alter a period of time to see whether the gap voltage is larger than a predetermined reference voltage or not. If the gap voltage is larger than the reference voltage, it means there is a proper gap between the wire electrode 4 and the workpiece 5, so that it will be deemed as normal discharge when it discharges next time. Otherwise, if the detected voltage is smaller than the reference voltage, it means that the distance between the wire electrode 4 and the workpiece 5 (gap) is too small, and it would be deemed as are discharge next time.
The wire cut electrical discharge machine(EDM) as above-mentioned judges first, whether the electrical discharge (of next time is normal discharge or, arc discharge by electrical discharge detecting circuit 7 alter initiating machining, then controls the driver circuit 9, by the timing signals produced by the discharge timing controller circuit 8 to control the turn-on of the set of switches 10 SW-1.about.SW-n for pulse width modulation, making the pulse width of normal discharge to be twice that of the arc discharge, so as to obtain a differert discharging energy for maintaining a stable machining. The discharging current signal waveform at point "a" of the circuitry is shown in FIG. 2, wherein T1 is the half cycle pulse width controlled by normal discharge and T2 is the half cycle pulse width controlled by arc discharge; the slopses of both discharge states are the same, S1; the provided energy ratio of normal discharge and arc discharge equals (T1/T2).sup.2, the energy variance during electrical discharge, therefore, is nonlinear and the wire electrode 4 breakage occurs easily due to the same large discharging slope of normal discharge and arc discharge, affecting the machining quality and speed.